High Temperature Superconductors are of great technological importance due to the fact that their transition temperatures can be greater than the boiling point of liquid nitrogen. High temperature superconductor materials, which are generally copper oxides, have been made in several forms, e.g., powders, single crystals, etc. Thin films of superconducting materials are useful in several applications including magnetometry and electronics.
There are many ways of depositing a thin film of material. In making films thinner than 1 .mu.m, vapor deposition methods are generally used. One such method is planar magnetron sputtering. In this method, a plasma is generated near the surface of a target, a large (e.g., 7.5 cm dia. .times.0.75 cm thick) disk of the material to be deposited. Typically, argon or another noble gas is used as the sputtering agent; in reactive sputtering, a partial pressure of reactive gas (the gas reacts with the target material to form the desired compound to be deposited) is used. The target is biased so that ions from the plasma are accelerated toward it. Ions hitting the target knock material, on an atomic scale, off of the surface. A magnetic field, generated by permanent magnets situated behind the targets, is used to localize and enhance the plasma near the target surface. The ejected material is collected on a "substrate" which is usually situated opposite and some distance from the target, typically just beyond the extent of the plasma. The substrate is often heated giving the adsorbed atoms enough surface mobility to arrange themselves into a crystal lattice.
Applying the sputtering technique to the deposition of oxide thin films was not straightforward. The problem is that oxygen ions (from a ceramic target or as a reactive gas necessary to form the oxide superconductors) are also generated by the plasma. The negatively charged oxygen ions are accelerated away from the negatively biased target and therefore toward the growing film. The oxygen ions then "sputter" away the film as it grows (negative ion bombardment). This phenomenon has two associated problems: 1) the growth rate of the thin film is strongly reduced, and 2) the oxygen ions, in some cases, preferentially sputter one element versus another leading to non-stoichiometric films.
The method of "off-axis" sputtering as shown in FIGS. 1A and 1B was developed to get around the negative ion bombardment problem. As shown in FIG. 1B this method consists of situating the substrate 1 not facing the target 2, but in the plane roughly perpendicular (about 70.degree. to 90.degree.) to the target, facing the plasma. As described above, Ar ions bombard the target (heavy arrows 3) generating a plasma. The plasma is contained by the field of the magnet assembly 4. In this case, the negative ions are not accelerated directly toward the growing film. Unfortunately, the growth rate of the film is substantially reduced as the momentum of the ejected material is also greatest directly out from the target (light arrows 5). Note that the arrows are meant to indicate overall flows, not individual atomic trajectories. Diffusion of the ejected material in the perpendicular direction is relied upon. Typically a higher pressure is used during off-axis sputtering. The higher pressure is also important in the formation of many of the copper oxide superconductors. Off-axis sputtering as generally practiced produces films with a large thickness gradient unless the substrate is rotated during deposition or multiple sputter sources are used.
A common element of nearly all "active" superconducting thin film circuits is the Josephson junction (JJ). The Josephson junction is a device which consists of two regions of superconducting material separated by a narrow region of non-superconducting material. In the Superconductor-Normal metal-Superconductor (SNS) Josephson junction, superconductivity is induced in the normal metal allowing a small supercurrent to flow through the metal without resistance. When the current through the JJ exceeds the "critical current" of the device, a voltage is generated across the device. This non-linearity can be used as a switch in an electronic circuit.
Fabrication of a JJ can be accomplished by depositing a superconducting film over a sharp step in a substrate and at an angle such that the step shadows an area next to it and the superconducting film is not continuous across the step. The necessity of depositing the film from one direction (directional deposition) precludes rotation of the substrate during deposition or the use of multiple sources arranged around the substrate.
The use of off-axis sputtering in making such devices is problematic because the thicknesses of the layers of the device strongly effect the device characteristics. The gas flow manifold of the present invention greatly improves the uniformity of the deposition and increases the overall growth rate.
The use of gas flow manifolds is known in thin film processing, for example, in reactive sputter deposition. If the sputtering rate of the reacted material is lower than that of the unreacted material, the partial pressure of the reactant gas is kept low at the target surface so that the sputter rate remains high, and at the same time, the reactant gas partial pressure is kept high near the substrate so that the material deposited is fully reacted. In such a case, the reactive gas may be admitted via a gas flow manifold as close as possible to the growing film on the substrate as in T. Jung and A. Westphal, Surface and Coatings Technology 59, 1993, pages 171-176 or in S. Maniv, C. Miner, and D. Westwood, J. Vac. Sci. Technol. 18, March 1981, pages 195-198. Alternatively, introduction of the non-reactive sputter gas as close as possible to the target is done in the commercial product, "A300" sputter gun with integral gas injection by AJA International, P.O. Box 246, 809 Country Way, North Scituate, Mass. 02060.
Thus a need exists for a sputter deposition technique which can produce a high deposition rate which is uniform over a large area and is suitable for growing both thin films and step-edge junctions. Applicants' invention provides such a deposition apparatus and process.